Prostate cancer is the second leading cause of cancer-related death in American men. Prostate Specific Antigen (PSA) has been widely used as a screening test for prostate cancer. However, due to its lack of specificity, additional serum biomarkers are needed. PSA is, in fact, highly expressed in benign prostate epithelia as well as prostate cancer. Large-scale gene expression profiling studies performed by our group and others have identified a host of markers preferentially over-expressed in prostate cancer epithelia relative to benign. Unfortunately, however, most of these biomarkers are not secreted, expressed at relatively low levels, and thus not detectable in serum. To overcome this, our group is investigating approaches to employ the body's endogenous immune response as a natural "amplification strategy" to detect carcinoma. There is a growing body of literature supporting the notion that cancer produces a humoral immune response in the host. Thus, characterizing and multiplexing this antibody repertoire for the purpose of early detection of cancer may have clinical utility. Our preliminary data provides compelling evidence that prostate cancer patients produce a humoral immune response to the prostate cancer marker, alpha-methylacyl CoA racemase (AMACR). While the immune response to AMACR represents an initial starting point, we expect that defining the entire antibody repertoire produced against tumor antigens will lead to highly specific and sensitive multiplexed assays for the detection of prostate cancer. Thus, our general approach is to harness the endogenous immune system as a biological "sensor" for cancer. Given this our Aims are as follows: Specific Aim 1: Characterize and validate the humoral immune response to AMACR in different patient cohorts. Specific Aim 2: Employ high-throughput phage epitope microarrays to identify candidate humoral response markers of prostate cancer. Specific Aim 3: Define and develop a multiplexed protein/epitope microarray to identify prostate cancer based on humoral response.